At present, a fuel cell system including a fuel cell for generating electric power by receiving supply of reactive gases (a fuel gas and an oxidation gas) to generate electric power has been proposed and put to practical use. When the electric power is generated by such a fuel cell system, moisture is generated in the fuel cell by the electrochemical reaction, and this moisture remains in reactive gas channels of the fuel cell, with the result that a flow of the reactive gas may be possibly hampered. When the fuel cell system is operated under an environment at a low temperature such as a temperature below the freezing point, the moisture remaining in electrodes (a catalytic layer and a diffusion layer) of the fuel cell freezes, and a starting performance remarkably deteriorates in some case.
As a conventional technology to solve various problems caused by the moisture generated in the fuel cell in this manner, a technology (a scavenging technology) is proposed in which dry oxygen or dry hydrogen is supplied to the reactive gas channels when the operation of the fuel cell is stopped to thereby remove the moisture from the fuel cell. In recent years, a technology is proposed to perform a scavenging operation in such a manner that a moisture content included in electrolytic films of the fuel cell is estimated, and a respective device (a compressor, a pump, etc.) is driven and controlled so that this estimated moisture content becomes a predetermined value (see Japanese Patent Application Laid-Open No. 2004-158274, for example).